Heres a quote from Mike McGrath (long ago editor of Organic Gardening magazine)
"Yellow leaves can also be a sign of overwatering, underwatering, and 147 different diseases BUT...most often these symptoms are the telltale traits of a beast that eventually pounces on all tomato growers: A wilt that builds up in soil where tomatoes are grown year after year. In this area, it's generally Verticillium, but I wouldn't be surprised if its ugly cousin from warmer climes, Fusarium, was enjoying our sultry summers.
This year: Remove yellow leaves promptly (wilted ones may recover when the sun goes down). Keep the plants well fed (but not overly) and watered (but not overly); healthy plants can outpace the disease with their new growth - especially in this weather!
Next year: Grow disease-resistant varieties (they'll have the letters "VF" after their names). Plant in soil where tomatoes have never grown (or grow in containers on top of the soil, or completely change the soil in infested containers). And use lots of compost (a natural disease-fighter) to try and keep the wilts at bay."
There are two other more simple possibilities. Did you harden your plants off sufficiently? Yellow leaves on a recent transplant can be due to transplant shock. Were they root-bound in their containers? This can cause stress, also. When the outdoors weather, (especially if its sunny and/or windy,) suddenly causes the leaves to require more water than usual and the roots can't keep up with the demand, the leaves will wilt, and eventually turn yellow if the stress continues. Colder nights than the plant is used to can cause this, too. Don't fertilize if this is the case. That just creates more stress. Wait until new growth starts. If it is healthy and green, no problem. You've just set your plants back some. If it is nasty looking, too, then you have another problem, possibly one of the wilts or lack of or over fertilization.
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